review article fundamental aspects of electropolishing

REVIEW FUNDAMENTAL

ARTICLE

ASPECTS

OF ELECTROPOLISHING

D. LANDOLT Materials Department, Swiss Federal Institute of Technology, 1007 Lausanne, Switzerland (Received

18 March

1986)

Abstract-The literature on the mechanism of electropolishing is reviewed. A brief historical outline is given and anodic levelling or macrosmoothing is distinguished from anodic brightening or microsmoothing. Macrosmoothing results from differences of the gradient of electrical potential and/or concentration on a rough surface. Its rate can be quantitatively predicted in many cases. Microsmoothing results from the suppression of crystallographic factors on the dissolution process. Optimum conditions arc achieved when dissolution is under mass transport control. The role of anodic films and of pitting is discussed.

INTRODUCTION

has progressed[38] and new experimental methods have become available for the study of surface films and passivation phenomena[39]. In the present paper the literature on fundamental aspects of electropolishing is reviiwed and an attempt is made to critically evaluate our state of knowledge in this area.

The discovery of electropolishing goes back to the beginning of the century[l, 21 but the first systematic investigations leading to practical applications are due to Jacquet who received a palrnt for the process in 1930[3]. Jacquet attributed the electropolishing effect to the presence at the anode of a “viscous layer” the exact role of which remained somewhat obscure[&6]. Elmore pointed out the importance of diffusion for electropolishing[7]. In the fifties Hoar and coworkers postulated that brightening requires the presence on the anode of a compact solid film[8-121. Later Hoar and Rothwell investigated the role of convective mass transport[13, 141. Further studies on surface phenomena and transport processes were carried out by Epelboin and coworkers who attributed the polishing effect to the existence of an anhydrous layer[l5-201. The first quantitative experimental investigations of the rate of smoothing are due to Edwards[21, 221. Wagner treated the same problem mathematically[23]. Electropolishing is normally carried out in concentrated acid media such as phosphoric acid, sulfuric acid and their mixtures or in perchloric acid-acetic acid solutions. Tables of electropolishing electrolytes for different metals can be found in the books by McTegari[24] and by Shigolev[25]. Methanol is sometimes used as a solvent instead of water[26-321. The development of electrochemical machining in the late fifties and early sixties showed that high rate dissolution of metals in neutral electrolytes under intense forced convection conditions may lead to electropolished surfaces[33], convective mass transport being the controlling factor[34, 351. More recently, it has been found that electropolishing in neutral salt solutions can be obtained at relatively low average current densities and flow rates if high intensity current pulses are employed[36, 371. From the time of publication of the classical reviews by Hoar[l&l2] our understanding of mass and charge transport processes in electrochemical systems

MACROSMOOTHING SMOOTHING

32:1-11

MICRO-

Electropolishing is defined in a general way by ASTM as the improvement of surface finish of a metal effected by making it anodic in an appropriate solution[40]. In the literature it is common to distinguish anodic levelling or smoothing from anodic brightening. The former refers to the elimination of surface roughness of height > 1 pm, the‘ latter to the elimination of surface roughness < 1 pm[24]. Surface brightening thus results from the elimination of surface roughness comparable to the wave length of light. The distinction between anodic levelling and brightening solely based on surface roughness is a simplification, however, because there is no simple relationship between profile height measured for example by means of a mechanical testing device and brightness determined by measuring the specular reflectance[41]. The latter is influenced by the shape of the surface profile characterized by the amplitude density funcand by the autocorrelation function[41]. tion, According to Hryniewicz et al. brightness is determined by the variance of the profile slope[42]. On anodically dissolved surfaces brightness essentially depends on whether dissolution leads to surface faceting (crystallographic etching) or not. Edwards introduced the terms macrosmoothing and microsmoothing for anodic levelling and anodic brightening, respectively[22]. The two processes are fundamentally different. Macrosmoothing results from the concentration of current lines on peaks of a surface profile thus leading to a locally higher dissolution rate. Theoretically, macrosmoothing is treated as 1

E.4

AND

2

D. LANDOLT

a current distribution problem in which the surface is assumed homogenous from a kinetic point of view. Microsmoothing on the other hand results from the suppression of the influence of surface defects and of crystallographic orientation on the dissolution process. Its theoretical understanding, therefore, requires knowledge of the mechanism of atom removal from the crystal lattice and involves the study of surface kinetics and of passivation behavior. In practice it is possible to achieve macrosmoothing without microsmoothing and vice versa. Figure 1, based on the work by Sautebin et al.[43,44], serves as an illustration. It shows a mechanically machined triangular surface profile made of copper before (a) and after anodic dissolution (b, c). In both cases anodic dissolution leads to a decrease in profile height, ie to macrosmoothing. Microscopically, there is a difference, however; (b) exhibits a crystallographically etched surface which appears dull to the eye, (c)

Fig. 1. Macrosmoothingand microsmoothingofa triangular surface protile. (a) Original profile of copper anode fabricated by mechanical machining. (b) Anode after dissolution in absence of mass transport effects showing anodic levelling and crystallographic etching. (c) Anode after dissolution at the limiting current showing anodic levellingand brightening.

exhibits a crystallographically smooth surface which appears bright to the eye. Only (c), therefore, has undergone both macrosmoothing and microsmoothing. For the dissolution conditions of (c) microsmoothing without substantial macrosmoothing can be achieved by minimizing the total amount of dissolved metal. The electrochemical conditions leading to macrosmoothing and to microsmoothing will be discussed in more detail in the following paragraphs.

RATE

OF

ANODIC

LEVELLING

The rate of anodic levelling or macrosmoothing is equal to the difference in dissolution rate between peaks and recesses of a rough surface. It is determined by the prevailing current distribution on the surface profile and, therefore, depends on both geometrical and electrochemical parameters including hydrodynamics. The first systematic investigation of the rate of macrosmoothing was carried out by Edwards[22] who studied the shape evolution of model profiles corresponding to spaced grooves of gramophone records. He showed that the highest rate of levelling is obtained under primary current distribution conditions but for geometrical reasons the employed profile is levelled even if the local current density is uniform, a situation he termed geometrical levelling. Wagner[23] calculated the rate of anodic levelling of low amplitude sinusoidal profiles. Wagner’s solution predicts an exponential decay of profile amplitude with dissolution time. More recently, McGeough[45] and Fedkiw[46] extended Wagner’s analytical solution to higher amplitude sinusoidal profiles but the theoretical treatment of more complex surface profiles and cell geometries requires the use of numerical methods such as the finite difference method[47], the finite element method[43, 44, 48-501 or the boundary element method[51]. These methods allow one also to take into account anodic overvoltage. We shall first discuss the influence of geometrical factors. It has been pointed out by McGeough[45] that an arbitrary two-dimensional surface profile can be described by a Fourier sine series where the individual Fourier coefficients behave independently. For the discussion of the influence of geometrical factors we may, therefore, refer to the sinusoidal profile shown in Fig. 2. The cell geometry is defined by three characteristic lengths, the amplitude e (or the initial profile height r0 = 2a), the wavelength i and the counter electrode distance 1.From these quantities two dimensionless parameters can be formed, for example s0/l and c,ll which characterize the influence of geometry on anodic levelling rate. The behavior is illustrated by Fig. 3[50] in which the charge (expressed in dimensionless form) needed to decrease the height of a sinusoidal profile to SO% of its original value is shown as a function of ~a/1and e,,j,l_ The values shown were calculated for primary current distribution using the finite element method (FEM). For comparison, the analytical solutions given by Wagner[23] and McGeough[45] for the limiting cases of 1~ I, &,,/A+ 0 and I P I, &,,/I+ 0, respectively, are also indicated. It follows from Fig. 3 and similar calculations[48] that the charge necessary to decrease the profile amplitude

Fundamental aspects of electropolishing

3

Fig. 2. Sinusoidal surface profile of wavelength I and profile height Ed.The straight solid line represents the counter electrode at a distance 1 or the outer limit of the Nernst diffusion layer of thickness 6. The broken line represents the outer limit of the Nernst diffusion layer for an ideal macrotile.

k 9‘. : I

1.2

I

Wagner

‘r

lsmusprofll

EJI

=O)

1

Fig. 3. Calculated charge (in dimensionless form) corresponding to a decrease in the height of a sinusoidal profile to 50 % of its initial value as a function of the geometrical parameters Q/I and .q,/A. Primary current distribution[50].

to a preset value is smaller if ~e/jl is large. For a given initial profile height anodic levelling, therefore, is the slower the larger the wavelength. This has practical implications. For example, during the elimination of distantly spaced grooves resulting from mechanical polishing the surface profile upon anodic dissolution transforms into a sinusoidal shape, the fundamental wavelength corresponding to the groove spacing. The resulting unfavorable value of the ratio Q/A makes it

relatively difficult to anodically level such a profile[48]. In most practical situations the distance of the counter electrode is much larger than the profile height and its influence can be neglected (asymtotic solution for EJI --t 0). However, if anodic levelling is carried out under ECM conditions, eg in deburring, the counter electrode distance may be smaller than the fundamental wavelength I and the parameter &,,/I becomes dominating[45]. The discussion presented so far on the influence of geometrical factors on the rate of anodic levelling strictly applies to primary current distribution, ie in the absence of significant contributions of charge transfer and concentration overvoltage. Charge transfer overvoltage (secondary current distribution) tends to reduce the rate of anodic levelling[44] but the influence of geometry is qualitatively the same. A measure of the relative importance of charge transfer overvoltage is the Wagner number, Wa = (drI/di)/@,e,) where dq/di is the slope of the current voltage curve, pe the electrolyte resistivity, .a0 the initial profile height[44]. In the presence of concentration overvoltage (tertiary current distribution) two cases must be distinguished. Below the limiting current both mass transport and potential distribution influence the current’distribution on the surface profile and hence the rate of levelling. This situation has not been treated in the literature up to now and is probably of minor practical interest. At the limiting current the current distribution is governed solely by mass transport. Anodic levelling under these conditions was treated theoretically by Wagner who used the Nernst diffusion layer model[23]. If the profile height is much smaller than the thickness of the stagnant diffusion layer (so called microprofiles, &,,/ad 1) the mathematical treatment is the same as for primary current distribution, Laplace’s equation for potential, V+’ = 0 being replaced by V2c = 0 where c is the concentration of the transport limiting species and the interelectrode distance 1being replaced by the diffusion layer thickness 6.

D. LANDOLT

0 DISTANCE

i

FROM

ANODE-

Fig. 4. Schematic representation of mass transport mechanisms involving the anodically generated metal ions M, (I), an acceptor anion A (II) or water (III) as transport limiting species. MA, represents a complex ion and C,,

is the saturation concentration.

Wagner assumed an acceptor to be rate limiting (see next section) but the mathematical treatment is valid for any transport mechanism. It is, therefore, not possible to test Wagner’s assumption concerning the transport limiting species by measuring the rate of decrease of profile amplitude as was recently claimed in the literature[52]. The theoretical prediction of the rate of levelling of microprofiles at the limiting current based on the Nernst diffusion layer model is in good agreement with experiment. This was shown by studying the rate of anodic levelling of triangular model profiles oriented parallel and perpendicular to the flow direction and comparing the results with numerical calculations by the finite element method[44, 491. The situation is more complicated in the case of macroprofiles, ie for profiles where EJB > l.* Ideally, if 6 Q e0 the diffusion layer should follow the surface profile in a perfect way as indicated by the broken line of Fig. 2. In such a case the current density is uniform and only geometrical levelling can occur. The situation is usually not encountered in practice because the surface profile perturbs the local hydrodynamics[49]. For example, anodic levelling experiments performed on triangular macroprofiles in a flow cell yielded a higher rate of levelling if they were oriented perpendicular rather than parallel to the flow direction[49,50]. A quantitative prediction of the rate of levelling in this case would require detailed modelling of the hydrodynamic perturbation introduced by the macroprofile. Such modelling using the finite element method has recently been applied to two dimensional profiles representing corrosion pits by Alkire et aI.[53].

MASS

TRANSPORT

LIMITING

SPECIES

In practical electropolishing applications one usually wants to achieve both macrosmoothing and microsmoothing. While macrosmoothing can be * The terms microprofile and macroprofile are defined with respect to the ratio eO/Sand not in terms of absoke value of profile height. Depending on hydrodynamic conditions, ie the value of 6, a given surface profile may, therefore, behave as a microprofile or a macroprofile.

achieved under ohmic or under transport control (see above) only the latter condition leads to microsmoothing. In the present paragraph the question of the transport limiting species is discussed. Three possible transport mechanisms have been proposed in the literature. They are schematically represented in Fig. 4. The salt precipitation mechanism I involves rate limiting diffusion of cations of the dissolving metal (M,,) from the anode into the bulk. At the limiting current a thin salt film is present on the anode and the surface concentration of M,, is equal to the saturation concentration. To maintain electroneutrality electrolyte anions (A) accumulate in the anodic diffusion layer. The mechanism II involves rate limiting diffusion of acceptor anions which are consumed at the anode by formation of complexes (MA,). Mechanism III involves the rate limiting diffusion of water from the bulk to the anode where it is consumed by formation of hydrated metal ions M,, In mechanisms II and III the surface concentration of the rate limiting species is zero at the limiting current. The actual concentration profiles at the anode are, of course, more complicated than those shown in Fig. 4 because of the presence of other species resulting from dissociation or association reactions, and because of viscosity changes. Furthermore, the Nernst model used here is only an approximation of real behavior. Most early investigations on the mechanism of electropolishing were carried out with the copperphosphoric acid system[4, 7, 13, 21, 541 and this metal electrolyte combination has remained popular for such studies [5563]. Copper in concentrated phosphoric acid exhibits a mass transport limited current plateau (Fig. 5) but the identification of the transport limiting species is not straightforward because of the comsolution chemistry plicated involved[%-581. According to Elmore[7] the rate limiting step in the dissolution process is the diffusion of Cu’ ’ ions away from the anode. At the limiting current the concentration of Cu2+ at the anode surface corresponds to the saturation concentration. According to this mechanism the value of the limiting current should decrease when Cu’+ ions are present in the bulk electrolyte because the concentration gradient between anode and bulk is reduced. This is in apparent contradiction with transient data obtained by Edwards[21] who found that the presence of Cua+ in the bulk did not markedly

5

Fundamental aspects of electropolishing 2coDenotes polishing

P

:g

start

of good

150-

4 : 5 % c t :: E e ; u

Ioo-

5c-

0

/

I

I

I

I

loo0 Anode

2&o

1500

potential

&/mV

Fig. 5. Anodic current voltage curves for copper in 6M H,PO, measured at different flow velocities by Hoar and Rothwell[13].

lower the transition time. Edwards, therefore, thought diffusion of acceptor anions from the bulk to the surface to be rate limiting (mechanism II). This, however, is in contradiction with the observation that the measured transition time decreases rather than increases with increasing phosphoric acid concentration as illustrated by Fig. 6 taken from reference[63]. Kojima and Tobias[56] evaluated different possible transport mechanisms by taking into account the variation of transport properties with composition determined by Krichmar et af.[59] and concluded that Edwards data are not inconsistent with Cua+ being transport limiting. Several other authors also concluded that Cu2+ is the rate limiting species[57, 59, I.C

I-

. 0

\

0

0.e I-

Edwards E lmore Kojima.Tobias a11 galvanostatis

* 0

%

This study. constant cell

voltage

(stage

II)

631. Such a mechanism was recently questioned by Glarum and Marshall[%] who postulated that water acting as an acceptor is the most likely transport limiting species (mechanism III). No proof for this hypothesis was given, however. According to Wagner only an acceptor mechanism can explain the observed limiting current plateaux covering a potential range of one volt or more[23]. However, Wagner’s argumentation does not take into account the possible presence of a surface film which may cause substantial potential drops at the anode. The mechanism of electropolishing of nickel in sulfuric acid has been investigated by a number of authors[ 14,18,64-66]. Electropolishing is achieved in the transpassive potential region at HaSO, concentrations 3 8 M where mass transport limited currents are observed[18,64,65]. The value of the limiting current density decreases with increasing acid concentration[64-661. This fact alone does not permit to distinguish between mechanisms I, II and III because the solubility of nickel sulfate, the concentration of free sulfate and the concentration of water all decrease with increasing acid concentration. Adding nickel sulfate to the bulk solution decreases the value of the limiting current[65]. This speaks in favor of mechanism I. The behavior is illustrated by the data of Fig. 7[65]. According to the Levich equation the quantity di,/d,/w (ir = limiting current density, w = rotation rate) for a mass transport controlled process is proportional to the concentration difference between bulk and anode surface. Adding NiS04 to the bulk solution decreases the relevant concentration difference and hence the value of di,fd,/w. The extrapolated intercept with the X-axis, in principle, permits one to estimate the concentration prevailing at the anode surface. In the case of Fig. 7 the concentration value estimated in this manner largely exceeds the saturation concentration of NiS04 in sulfuric acid. According to Alanis and Schiffrin[65] this indicates that the water concentration at the surface is higher than in the bulk. A more likely hypothesis is that a metastable phase is formed at the anode, consistent with the observation that in concentrated HzS04 supersaturated solutions of NiSO, can easily be produced in the laboratory[67]. However, a definite proof for the existence of such phases and a quantitative interpretation of measured limiting currents in very concentrated acid media is still lacking.

,-

03

I

I

I

0

7

9

Concentration

of

5

H,

I

II

13

PO, /

mol

I

I5

C,,/ml-’

C’

Fig. 6. Influence of H,P04 concentration on transition times (i ,/;) for copper dissolution[63].

Fig. 7. Dependence of the slope di/d ,/w (i = limiting current, w = rotation rate) on bulk nickel sulfate concentration. Nickel dissolution in 9.25 M (0) and 11.5 M (I) HzS04[65].

D.

6

LANDOLT

Recent studies with rotating disk electrodes provide additional support for mechanism I. Heinrich and Feller[31] showed that addition of Ni’+ to the bulk reduced the value of the anodic limiting current for nickel dissolution in methanolic 2 M H,S04 in a linear manner. Similarly, saturation of a H3P04-H2S04 electropolishing electrolyte with CrO, suppresses the limiting current for chromium dissolution in the transpassive potential range[68]. Finally, electropolishing of iron and cobalt in methanolic H,SO, and in concentrated H,PO, have recently been found to involve mass transport of reaction products and salt precipitation at the surfaceC32). Anodic limiting currents obtained in aqueous salt solutions are generally easier to interpret quantitatively than those observed in very concentrated acid media. For example, iron in 4M NaCl exhibits well defined limiting currents[69] which on a rotating disk electrode vary linearly with the square root of rotation rate. Their value decreases with increasing chloride concentration. This together with the high water concentration of such solutions allows one to definitely rule out mechanisms II and III. Experiments performed in a binary FeCl, electrolyte show an almost linear decrease of the limiting current with bulk FeCl, concentration in agreement with mechanism 1[69]. The intercept with the X-axis corresponds quite well to the saturation concentration (Fig. 8). The same behavior is observed for nickel[70,71]. Dissolution of both metals at or above the limiting current leads to surface brightening[72]. Mass transport of dissolved metal ions is the limiting factor for the change from surface etching to surface brightening during high rate dissolution of copper in nitrate and sulfate electrolytes[73, 741 and of iron and nickel in nitrate and chlorate electrolytes[75-777. In all these cases the estimation of the surface concentration of dissolving metal ions at the limiting current yields values in

ZO#-

4

ROTATION RATE

0 v

6.0

CONCENTRAT,ON

OF

1600 1200

rpm l-pm

0

600

rpm

0

400

rpm

FERROUS

~~~~~l)

Fig. 8. Anodic limiting currents for iron dissolution in FeCl, as a function of bulk concentration[69].

reasonable centration.

agreement

ANODIC

with

the

saturation

con-

BRIGHTENING

The morphological difference between a crystallographically etched and a bright surface is illustrated by Figs lb and lc. In this section the electrochemical conditions leading to the two types of surface structure are examined. When a metal crystal dissolves in the active potential region the rate of dissolution at a given potential depends on orientation. For example, on copper which has an fee structure the [lOO] planes have the lowest, the [ill] planes the highest dissolution rate[78]. A copper single crystal sphere upon dissolution, therefore, transforms into an octahedron with its flat sides parallel to the [l 1l] plane[78, 791. In a similar way anodic dissolution of polycrystals or monocrystals of random orientation leads to faceting, ie formation of crystallographic etch patterns revealing distinct crystal planes. The behavior is explained by the classical theory of crystal growth and dissolution which postulates that dissolution involves removal of atoms from energetically favored kink sites on monatomic steps situtated on close packed planes. The resulting surface morphology depends on the rate of nucleation and on the lateral velocity of the steps[8&83]. For example, a [ 11 l] oriented defect free silver monocrystal remains perfectly flat if nucleation of monatomic steps is slow compared to their lateral velocity[80]. On arbitrarily oriented crystals or in presence of a high density of imperfections nucleation of monatomic steps is fast. Monatomic steps moving at different velocity may bunch leading to the formation of microscopically observable etch patterns[81]. Bunching may be regarded as a purely statistical process but in practice it is strongly affected by adsorbtion of anions or other species[81, 831. The described kink-step dissolution mechanism operates in the active dissolution region below the limiting current. It leads to surface etching even if dissolution rates are extremly high[84]. There is evidence that the same mechanism holds in the transpassive potential region under certain conditionsC64, 68, 857. For example, during transpassive chromium dissolution in a HJPO,-H2S04 electropolishing solution crystallographic etching was observed below the limiting current[68]. In cathodic metal deposition bright deposits are achieved by working in presence of suitable inhibitors[86]. Anodic brightening on the other hand, is usually attributed to the presence of surface films[8, l&12]. According to Hoar the dissolving surface must be covered by a “thin compact solid film” consisting of an “oxide contaminated with significant amounts of anion from solution”[ll]. The ionic conductivity of the film must be sufficient to allow for the passage of anodically formed cations at high rate. Removal of atoms from the metal into the solid film occurs randomly since it is governed by the availability of cation vacancies in the film and not by the lattice position of the metal atoms[ll]. Experimental evidence for the presence of a thin solid film on the anode

Fundamental aspects of electropolishing

under electropolishing conditions was obtained by Hoar and coworkers from wetting experiments with mercury on polished and non polished metal surfaces and from optical, coulometric and impedance measurements[ll]. Novak et ~I.[871 used ellipsometry to investigate anodic films formed during electropolishing of copper in phosphoric acid. The viscous diffusion layer was displaced with glycerol for this purpose and the surface studied without giving it any cleaning treatment. The obtained data suggested a film thickness of 4@120A independent of potential. If the electrode was rinsed with water, only a 20A air oxidation film could be observed. The behavior clearly suggests that the anodic film present during electropolishing is a water soluble compound and not an insoluble oxide. The ellipsometric data by Novak et al.[87] are only partially in agreement with results of impedance measurementsC58, 887. The latter yield the same order of magnitude for the film thickness on copper but they suggest that it varies with potential[58]. Unfortunately, under electropolishing conditions neither ellipsometric nor impedance data lend themselves easily to unambiguous interpretation. The physical nature of the films present on copper undergo&g electropolishing in phosphoric acid, therefore, is still not well known, some authors favoring a solid oxide type film[87-891, others a highly viscous anhydrous film[58]. The presence of anhydrous films at the electrode surface during electropolishing was originally advocated by Epelboin and coworkers[15, 16, 19, 203 who used polarized light to study anode surfaces undergoing electropolishing in perchlorate based electrolytes. A doubly refracting layer was observed which disappeared immediately upon current switch off indicating that it consisted of precipitated salt crystals[16]. Stoichiometric experiments performed with a number of different metals showed that many of them upon electropolishing in perchlorate containing electrolytes yielded abnormally low valencies, examples being Al+, Be+, Cu+ or Ti+[16]. This behavior was attributed to the existence of an anhydrous environment at the anode. However, more recently abnormal valencies were also observed in conventional aqueous perchlorate solutions at high dissolution rates[90]. Dissolution with abnormal valencies involves specific reactions of the perchlorate ion which are not necessarily restricted to anhydrous environments. In the process the perchlorate ion is reduced to chloride[lb, 917. Perchloric acid-acetic acid based electropolishing electrolytes frequently require the application of unusually large cell voltages[24] which cannot be explained by the resistivity of the electrolyte. The problem has been studied for titanium which can be electropolished in this type of electrolyte at anode potentials in excess of 20 V[91,92]. It was found that such high potentials are necessary to cause breakdown of the anodic oxide film which covers the metal at lower potentials. The behavior is illustrated by the Auger electron spectroscopy data of Fig. 9 which show an almost linear increase of film thickness with potential then an abrupt decrease at approx. 2OV[92]. Above this value, in the electropolishing region the measured film thickness is comparable to that of the natural oxide film formed in air. It cannot, therefore, be decided from these data whether the observed film was

7

400-

3

300.

i z

3

z 200. c

3

h

100. 0

o- . 0

10 ANODE

20 POTENTIAL

SO (VOLTS)

Fig. 9. Apparent thickness, determined by Auger electron spectroscopy, of oxide fkns on titanium anodically polarized at different potentials in a perchloric acid-acetic acid electropolishing electrolyte[92].

formed in solution or during sample transfer after the experiment. The observed presence of adsorbed chloride ion at the film-metal interface[92] would suggest the latter. The examples given illustrate the difficulties encountered in the characterization of electropolishing films. It is therefore not surprising that their physical properties are not yet well known. Recently, Heinrich and Feller investigated the electrical response of anodic films on nickel during electropolishing in methanolic H2S04 by applying small potential steps and measuring the resulting current transient[31]. The authors concluded that the films behaved as an ohmic resistance, its value decreasing with increasing rotation rate of the disk electrode employed. The calculated field strength was of the order of 10’ V cm- 1 which seems too high for a film behaving ohmically. The exact chemical composition of the films was not known but sulfate and nickel ions were believed to be the main constituents[31]. Beck and Rice[93] studied the conduction mechanism of salt films formed on silver in chloride media. They found that at low chloride concentration conduction occurs through pores and at high chloride concentration by a high field solid state mechanism. Glarum and Marshall[SS] interpreted their impedance data on copper dissolving at the limiting current in phosphoric acid by postulating that a space charge layer is formed at high potentials, its thickness being of the order of 10 nm. The properties of the surface films under electropolishing conditions depend on potential. This follows from impedance measurements on copper[58] and nickel[94], and from the observation that in the plateau region current oscillations may occur[58,68]. Also, the resulting surface finish at the limiting current may vary with potential. For example, according to Hoar and Rothwell[13] good electropolishing on copper under the experimental conditions of Fig. 5 occurred only at potentials above those indicated by the small triangles in the figure. A further indication comes from Auger electron spectroscopy analysis.

D. LANDOLT

8

During anodic polarization of chromium in H3P04-H,S04 sulfur was detected on the surface after polishing at potentials above the current oscillation region but not below, although the reaction was mass transport controlled in both cases[68]. The presence of a current plateau associated with an anodic film on the surface is not a sufficient criterium for microsmoothing, rather a necessary condition is that the reaction rate is mass transport controlled. For example, transpassive nickel dissolution in sulfuric acid yields well-defined current plateaus but only in a certain concentration and temperature range are they mass transport controlled[l& 64, 653. Best microsmoothing is achieved under these conditions. A similar behavior is observed with chromium and stainless steels in H,P04-H2S0J68, 953. A practical search for electropolishing electrolytes or for favorable experimental conditions, therefore, is best started with a determination of current voltage curves under controlled mass transport conditions, eg by using a rotating disk[29).

PITTING Pitting is a local attack of an otherwise passive metal induced by certain anions, such as chloride, under the influence of a high anodic potential. To achieve uniform electropolishing pitting must be avoided. The relationship between passivity, brightening and pitting has been discussed by Hoar[ll, 121 who stressed the importance of the anion/water ratio in the electrolyte and of the applied potential. According to Hoar a high value of the anion/water ratio favors uniform breakdown of the passive film over pitting and thus leads to surface brightening. Indeed, most commonly used electropolishing electrolytes have a relatively low water content (the previously mentioned salt solutions used in electrochemical machining are a notable exception). Hoar’s mechanism is intuitively appealing but it does not really explain why surface brightening rather than surface etching takes place under these conditions. Indeed, it is well known that the interior of corrosion pits may be crystallographitally etched or bright depending on conditions. Bright pits usually are associated with the formation of a salt film within the pit[9698]. It would, therefore, appear that uniform breakdown of the passive film by a pitting type mechanism is not a sufficient condition for surface brightening but that in addition prevailing conditions must permit formation of a salt film. The described behaviour is consistent with results from the study of high rate transpassive dissolution of iron and nickel using surface analytical techniquesC90, 1001. At sufficiently high potentials the passive film breaks down by a pitting type mechanism but the resulting dissolution leads to uniform etching. Only when the mass transport limited current for salt precipitation is reached does surface brightening take place[lOl]. Depending on the metal-electrolyte system considered anodic dissolution in the transpassive potential region below the limiting current leads to crystallographic etching or to pitting. Transpassive dissolution of chromium in H,PO,-H2S0, involving oxidation to soluble chromate is an example of the first case[68]. Stainless steels in the same electrolyte dissolve by

pitting but at the,limiting current the surface is smooth. The behavior is illustrated by Fig. 10 which shows surface micrographs of a type 304 austenitic stainless steel surface dissolved in the transpassive potential region below (a) or at (b) the limiting current[95]. The difference in surface morphology can be rationalized by considering the role of applied potential and of mass transport on pit nucleation and growth. Pits nucleate by passive film breakdown and apparently the number of nuclei increases with applied potential[W]. Pit growth depends on the relative dissolution rate within and outside of the pit, it proceeds both in depth and laterally. Eventually due to build up of dissolution products salt film precipitation oceurs[97] and the dissolution rate within the pit becomes mass transport controlled. This condition is established faster at higher potentials because of a faster rate of charge transfer. From a mass transport point of view pits are less favorable than a flat surface. For example, if dissolution within the pit and outside the pit are both mass transport controlled anodic levetling occurs by the same mechanism described above for arbitrary microprofiles[103]. Usually during pitting the surface outside the pit is covered by the passive film and therefore dissolves at a slow rate (Fig. lOa). However, if at higher potentials the pit nucleation rate is sufficiently high a condition may be reached where lateral merging of small pits dissolving under mass transport control leads to uniform dissolution behavior of the electrode and to a smooth surface finish. This condition corresponds to the limiting current plateau in the current voltage curve (Fig. lob). It follows from these arguments that the mechanism of anodic levelling and brightening in electropolishing systems involving transpassive film breakdown by pitting is fundamentally the same as that found in systems which

Fig. 10. Scanning electron micrographs of the surface of a stainless steel after transpassive dissolution in a H3POI-H,SO+ electropolishing electrolyte below the limiting current (a)and at the limiting current (b), respectively[95].

9

Fundamental aspects of electror,olishing exhibit crystallographic etching when dissolved actively or transpassively below the limiting current. The principal difference lies in the fact that in pitting systems the establishment of electropolishing conditions is intimately related to the breakdown behavior of the passive film and therefore requires a sufficiently high anodic potential. Non metallic inclusions are known to favor pitting in corroding systems by facilitating local breakdown of the passive film[ 1027. In electropolishing they can lead to pit formation by dissolving preferentially or by perturbing the local current distribution. Voids in the bulk metal or associated with inclusions also may cause pit formation during anodic dissolution. A further source of pitting is anodically evolved gas bubbles which adhere to the surface[24] or which nucleate at certain preferred locations perturbing the local diffusion layer[lO4]. In the latter case tails may be observed resulting from the stirring action of the gas bubbles[ 1041. Similar tails or flow streaks may appear under forced convection conditions in absence of gas bubbles. They result from local perturbations of the hydrodynamic flow caused for example by inclusions or pits[l03].

CONCLUDING

REMARKS

In the present paper the literature dealing with fundamental aspects of electropolishing has been reviewed and an attempt has been made to identify the electrochemical mechanisms responsible for macrosmoothing and for microsmoothing. It has been found that macrosmoothing results from local differences on a rough surface of the gradient of either the potential or of the concentration of the transport limiting species. The rate of macrosmoothing can be calculated for many situations involving well-defined cell geometries and/or the Nernst diffusion layer model by considering the current distribution on the surface profile. The mathematical analysis shows that if surface kinetics plays no role or if the surface behaves uniformly on a microscopic level short wavelength roughness disappears first, ie macrolevelling is preceded by microlevelling. This situation apparently prevails when dissolution is mass transport controlled, ie at potentials corresponding to the limiting current plateau or at higher potentials where the plateau is masked by concurrent oxygen evolution. The value of the limiting current in most if not all electropolishing systems is governed by the rate of transport of dissolution products from the anode into the bulk. During high rate dissolution in aqueous salt solutions the estimated surface concentration of dissolution products under these conditions agrees well with the saturation concentration of the corresponding metal salt. In concentrated acid type electrolytes the estimated surface concentration largely exceeds the value of the equilibrium saturation concentration, indicating that metastable species are formed at the surface. There is considerable evidence that a salt film is present on the anode at the limiting current. Its thickness and physical properties are not well known but they apparently depend on potential. Since the film disappears upon current switch off it cannot directly be

observed with ex situ methods. Whether the salt film as such plays a decisive role for microsmoothing is not entirely clear. Indeed, from a mathematical point of view, mass transport control of the dissolution process is a sufficient condition to achieve microsmoothing. On the other hand, the presence of an ionically conducting film could accentuate the microsmoothing process, for example by presenting a barrier that suppresses crystallographic effects in the sense described by Hoar[ll, 123. The observation that surface finish in the limiting current plateau region may vary with potential speaks in favor of such a hypothesis. Many electropolishing electrolytes used in practice contain a limited amount of water and some confusion exists in the literature as to the reason for this. High rate dissolution studies performed in aqueous salt solutions clearly show that a low water concentration in the bulk electrolyte is not a prerequisite for electropolishing. Rather, the available literature suggests that mass transport control of the dissolution process is the critical factor not only under conditions of high rate dissolution but also in conventional electropolishing. In the latter case a low water concentration may contribute to the establishment of mass transport controlled dissolution conditions in different ways. It renders more difficult the formation of passive oxide films which are subject to pitting. Furthermore, it reduces the value of the limiting current by decreasing the saturation concentration of anodically produced metal ions and in certain electrolytes such as phosphoric acid, by increasing the viscosity. Electropolishing is being used in many industrial applications. The present review shows that certain aspects of the process are now well understood and can be modelled quantitatively. Other aspects such as the properties of anodic films and of highly concentrated polishing media need further study. At present, it is not possible to formulate electropolishing electrolytes from theoretical principles. On the other hand, the concepts presented in this article can provide a rational basis for the planning of experimental programs aimed at the development and optimization of practical electropolishing processes.

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